1. Field of the Invention
This invention relates to Global Positioning Systems (GPS) and, more particularly, to techniques for providing differential range error information and range-rate error information.
2. Prior Art
The Global Positioning System (GPS) is a navigation system consisting of a constellation of 24 satellites in 6 orbital planes that provides accurate 3-dimensional position and velocity information as well as precise time to users anywhere in the world 24 hours a day. Each of the Satellites transmits on the same L-band frequencies (1575.42 MHz for the Standard Positioning Service) using independent Psuedo Random Noise codes for spread spectrum modulation. Satellite data includes system status, ephemeris and clock characteristics and is NRZ modulated onto the satellite carrier at 50 bits/sec.
User receivers measure their apparent range to the satellites by processing the received signals to determine transit time and correcting for atmospheric delay using stored and broadcast models. Since the location of the satellites, at the time of signal transmission, is known from the broadcast ephemeris the location of the receiver can be triangulated from the range measurements. The receiver's local clock error can be eliminated from the solution by incorporating one more satellite's range measurement than the number of dimensions being solved.
Similarly the receiver's velocity can be solved by comparing the measured Doppler shift of the received signals to the expected Doppler shift based on the satellites velocity vector, calculated from the ephemeris and projected on the line of sight to the satellite. After the user receiver's clock error is eliminated, by over determination, the residual Doppler is attributed to the user velocity.
Differential GPS offers the potential of accuracy's of 2-10 meters for dynamic applications and better than 2 meters for static applications. By placing a reference receiver at a precisely surveyed location and comparing measured GPS satellite ranges with ranges predicted by the satellite data transmissions, errors in the measurements can be computed. The errors in range and range rate are provided to a GPS receiver using errors signals encoded using the Recommended Standards for Differential Navstar GPS Service published by the Radio Technical Commission for Maritime Services (RTCM) Special Commitee No. 104. The Standard is called the RTCM SC-104 Standard.
This differential correction technique is effective only if the dominant error sources are receiver independent. For GPS users that are in reasonably close proximity the error sources are primarily selective-availability errors, atmospheric-delay errors and satellite-data errors. Selective Availability is a technique used by the Department of Defense that introduces artificial errors at the satellite for strategic reasons. DOD authorized users have access to keys that enable them that remove these error completely. Ionospheric and Tropospheric delays vary with the time of day, atmospheric conditions and Satellite elevation angle. Errors in the satellite's transmitted ephemeris and clock characteristics contribute to range measurement error. These error sources are all independent of the user's receiver and can be considered as common mode errors.
Under the RTCM SC-104 Standard, the error message structure for each satellite is 53 bits long. The RTCM system transmits all of the satellite data. Eight satellites in view of a reference receiver requires a data rate of approximately 400 bits/sec. for a system operating under the RTCM SC-104 Standard The RTCM SC-104 Standard uses sixteen bits for pseudo range correction. Eight bits are used for range rate correction. One bit is used for a scale factor. Thirteen bits are used for a time tag which provides 0.6 second increments for an hour period. Five bits are used for identification of a particular satellite. Eight bits are used for an Issue of Data (IODE) batch number. Two bits are used for a receiver station's estimate of the differential range error.
Information under the RTCM SC-104 Standard typically uses a transmitter which broadcast information over a relatively large area. For commercial broadcast transmitters, use of an auxiliary data channel with a data rate of 400 bits/sec. is relatively expensive. Consequently the need has arisen for a technique to economically and efficiently transmit differential GPS error information at a lower data rate, while still maintaining accuracy.